In my U.S. Pat. Nos. 3,845,941 and 3,971,666, there are described processes and apparatus for recycling used asphalt-aggregate compositions. That apparatus, of which the present invention constitutes an improvement, includes an elongated cylindrical drum having a plurality of heating tubes extending substantially along the length of the drum interior. Used asphalt-aggregate compositions are recycled by introducing them into an input end of the drum and rotating the drum while heating the tubes so that the composition becomes heated gradually by contact with the heated tube surfaces. The description in my aforesaid applications of the apparatus and method of its use in producing recycled asphalt-aggregate compositions are incorporated herein by reference.
Previous methods of heating asphalt compositions utilizing tumbling action within a cylinder such as described in U.S. Pat. No. 3,423,222 are inefficient since the materials are directly exposed to flame emitted from a burner. Not only does such exposure to direct flame or hot gases cause burning and deterioration of asphalt since the only portion of the material being heated is that exposed on the surface of the mass, but the resulting smoke and vapors drawn into the atmosphere are obviously a pollution hazard. In my previous co-pending patents this disadvantage is eliminated by utilizing heating pipes which not only avoids contamination in burning of the asphalt, since there is no exposure to flame, but greatly increases the heating efficiency since a significant amount of material is in contact with the hot tubes. Moreover, complete mixing of the composition is greatly facilitated because the pipes act to deflect the material in different directions cascading over the hot tubes as the drum rotates thereby improving the compositions's homogeneity.
In my U.S. Pat. No. 3,845,941, there is disclosed means for venting moisture or water vapor given off during the heating process from the input end of the cylindrical drum where temperatures are coolest. In addition, there also disclosed means for venting hydrocarbon gases volatilized from the heated asphalt composition at the hot output end and directing the gas into the heating or combustion chamber.
Notwithstanding the above-mentioned improvements in both the apparatus and process techniques there have remained certain problems which are to be solved according to the present invention. It has been found even with the addition of required amounts of make-up asphalt to the recycled material, in many cases, the asphalt present in the used recycled material has lost so much of the light hydrocarbons originally present that penetration and viscosity characteristics of the composition have been greatly affected. Thus, further improvement of the recycled asphalt-aggregate compositions is achieved by adding petroleum hydrocarbons during the heating and mixing process whereby original viscosity and penetration characteristics are obtained without weakening the material.
Still another problem is in venting of hydrocarbon gases from the output end of the apparatus where the recycled composition is heated to its highest temperatures in the process. Where the used material being recycled contains significant amounts of water, even though some water vapor is vented at the relatively cool input end of the apparatus much of the moisture remains in the material and is carried forward in the process, being vaporized nearer the output end thus using significant amounts of heat in the water vaporization. It is also desirable to remove this water vapor from the hydrocarbon gases vented from the heating cavity and directed to the combustion chamber.
In both of my aforesaid patents the elongated cylindrical drum in which the asphalt-aggregate composition is mixed and heated, a heating chamber at the forward or output end of the apparatus. A front end wall of the cylindrical drum forms one wall of heating chamber into which a flame from a gas or other fuel burner is directed. In addition, the heating tubes have open ends extending from this front wall and communicating with the heating chamber so that the tubes and front wall itself become extremely hot. This causes loss of hydrocarbon volatiles and even excessive burning or coking of the asphalt at the output end of the cylindrical drum from contact with extremely hot front wall and tube surfaces adjacent the front wall. It is to the elimination of these disadvantages and problems that the present invention is directed.